BUB for short. It is what you call the girl that you are dating or messing around with that isn't your first choice or your second girlfriend
When Speaking in front of your BUB. Use the word bubby so that she feels it is a term of endearment.

Rick: So How are you and Patty?
Jon: Well you know she is my back Up Bitch (BUB). Waiting for Jen to be single